1. The Field of the Invention
The present invention involves a device formed on a semiconductor substrate. More particularly, the present invention relates to a gate electrode stack with a refractory metal silicon nitride diffusion barrier.
2. The Relevant Technology
Modem integrated circuits are manufactured by an elaborate process in which a large number of electronic semiconductor devices are integrally formed on a semiconductor substrate. In the context of this document, the term "semiconductor substrate" is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term "substrate" refers to any supporting structure including but not limited to the semiconductor substrates described herein.
The semiconductor industry has been characterized with a continuous effort directed toward reducing device size and increasing the density of devices on semiconductor substrates. Such devices include transistors, capacitors, diodes, and the like. Regarding transistors, it has proved advantageous to use a refractory metal in the formation of gate electrode stacks and interconnections. Gate electrode stacks are conventionally designed with a thin layer of gate oxide positioned on a semiconductor substrate. A conducting layer of polysilicon is formed over the gate oxide. A second conducting layer consisting of a refractory metal, often tungsten, is formed over the polysilicon layer.
It has been found that a diffusion barrier positioned between the refractory metal layer and the polysilicon layer is useful for preserving the conductive qualities of the refractory metal. Processes used for producing semiconductor devices typically involve subjecting a device to several thermal processes with temperatures higher than the temperature at which refractory metals react with silicon to form silicides. For example, tungsten reacts with silicon at about 550.degree. C. to 600.degree. C. In the absence of a diffusion barrier, refractory metal material from the refractory metal layer diffuses into the polysilicon layer during thermal processes and forms a refractory metal silicide. The resulting refractory metal silicide increases the sheet resistivity of the semiconductor device, and causes the speed of the semiconductor device to be reduced. A diffusion barrier preserves the conductivity of the semiconductor device by preventing refractory metal from diffusing into the polysilicon layer during thermal processes.
Conventionally, diffusion barriers have consisted of either a refractory metal nitride or a refractory metal silicide. There are disadvantages to both of these materials. When a semiconductor device containing a refractory metal nitride diffusion barrier is exposed to a heat process, such as a source/drain reoxidation process, some of the refractory metal nitride is oxidized. Refractory metal nitride oxide has a greater bulk resistivity than refractory metal nitride. Therefore, the oxidized diffusion barrier impairs the conductivity and the efficiency of the semiconductor device. If sufficiently oxidized, the diffusion barrier can substantially electrically insulate the refractory metal layer. This can cause what is termed "a floating gate". Diffusion barriers made of refractory metal nitride have proven unsuitable in many applications involving thermal oxidation processes.
Likewise, problems are associated with diffusion barriers containing refractory metal silicide. Thermal processes can cause refractory metal silicide in diffusion barriers to form grains with grain boundaries. Refractory metal from the refractory metal layer can then diffuse through the diffusion barrier, facilitated by the grain boundaries, into the polysilicon layer. Some of the refractory metal layer can then react with polysilicon, thereby forming more refractory metal silicide, which has a bulk resistivity higher than that of bare refractory metal. Therefore, heating processes such as annealing can increase the sheet resistivity of semiconductor devices having refractory metal silicide diffusion barriers and can impair their function. Moreover, as device size is reduced, diffusion barriers containing refractory metal silicide impairs semiconductor device conductivity even in the absence of thermal processes.
As can be seen, it would be advantageous to provide a diffusion barrier that does not impair the conductivity of a semiconductor device as a result of a heating process. In particular, a diffusion barrier is needed that will not substantially oxidize during thermal oxidation processes. Additionally, a diffusion barrier is needed that will substantially prevent diffusion of refractory metal into polysilicon during heating processes.